Cycling UK Forum

531colin wrote:...forward is never a difficulty.

Never?
It's never a problem, providing you have the correct seatpost.

If my bikes were more laid back, I would need the saddle more forward than the seatpost would allow .......... despite a clamp for the saddle pack.
I've already said that the Brooks Team Pro has only 10mm forwards or backwards.

Maybe the saddle and seatpost should be a combo purchase ............ laid back, straight, or laid forwards?

Took the Brooks Team Pro off the Moulton.
The parallel bits of the saddle rails are 60mm long.
The clamp is 40mm long, and measuring Mercian's seat pin clamp, that one is 40mm too.
That means that a Brooks Team Pro has a maximum 20mm fore/aft movement. ie 10mm either way from central.

Mercian @ 73deg is 58mm behind the BB.

I also have a bike with a 73deg seat angle and the saddle about the same height, maybe a touch higher, as yours. The tip of my saddle is just over 75mm behind the BB, and I am using an inline post - the saddle was rammed right forward on the previous (traditional Campag style) layback seatpost. The difference, of course is the position and length of the parallel parts of the saddle rails. My saddle is a Specialized Toupe and it has approx 75mm of "clampable" rails. Unfortunately, I no longer have a Brooks Pro to compare with, but I am pretty sure the Specialized rails are quite a bit further forward than Brooks.

I have just measured an SLR and the parallel parts of its rails are almost the same length and position as the Specialized. I also have a UnicaNitor and the parallel parts of its rails are a bit shorter than the others and they are a good 20mm further back.

I wonder if these differences are common? I.e. modern saddles generally having longer and further forward rails than traditional ones. These differences easily make up for a degree or so of seat tube angle.

Maybe the saddle and seatpost should be a combo purchase ............ laid back, straight, or laid forwards?

Some uniformity in the design of saddle rails would be a good start!

Yep.
Spot on.

I'm lucky I guess.
Both bikes fit me despite one saddle being 10mm (or so) further back ............ and I didn't notice.

Now that I know, I may do something about it.
Trouble is, the 'bars will be 10mm too close.

Looks like I need to move both saddles 5mm ....................... so is it worth it?

In the interests of experiment I've switched saddles tonight. Put on a Charge Scoop the rails of which are a little shy of 90mm compared to the Pro at 60 mm. More importantly the distance from sit bone dimples to front of rail is 15 mm longer with the Scoop.
It's enabled me to get that 15mm back Colin suggests might give me the feel of pulling up on the bars. If it's dry in the morning I'll give it a try. I haven't a 10 mm shorter stem kicking about so I hope that doesn't alter the feel?

I'm much more sensitive to changes in saddle height than anything else. I'm driven to change my saddle height a few mm when I wear cycling boots,,,the soles can only be mm thicker. Its mad, but I can't stop myself doing it.
I don't know that I would notice 5mm fore and aft with the saddle, i suspect you can sit 5mm differently without knowing.
I'm also not sure i would notice 10mm more or less on reach, if the saddle was in the right place, although if the bike is much too short, I find I'm sitting on the back of the saddle, even if the saddle itself is in the "right" place,
YMMV!!

Pages 6 and 8 of Rivendell Reader #43 (100+ MB PDF file) talk about steep seat tubes on small frames. Grant Petersen seems to share 531colin’s view that steep angles on small frames are unjustified and a design error. He gives two possible reasons for this error on the two pages.

Interesting thread. I like that a question about saddle comfort is solved by foot position!

It's also prompted me to measure some things on my bikes. I'm happy with my foot position and my saddles are comfortable but there's a tangible difference in reach between them and even before I got the second, I sometimes had the feeling the first was a tad too long - though certainly not always. Having measured from tip of (identical) saddles to hoods, I find the 'long' bike has 6cm more reach than the 'short' one! And that's just measuring in a straight line, before I take into account the slightly wider handlebars.

I also tried the sitting on a stationary bike and looking at legs, along the lines of 531colin's posts and links. Surprising how visible the change to knee angle is when sliding backwards and forwards on the saddle; or perhaps I should say how little change is visible and tangible to hip angle (none, in fact). Though since I don't have a turbo-trainer, I had to do this (indoors) by leaning one shoulder against a wall and jamming the brake on with that hand, so probably not a very realistic reproduction of on the road riding.

Just had a tinker with the saddle, the seat post, and the Topeak clip.

Firstly, I removed the clip and moved the saddle as far forward as it would go ......... a whole 5mm.
Secondly, I gave the clip a good looking at.It seemed to me, that I could grind off the area that abuts the saddle clamp so it could be nearer to the saddle clamp .......... allowing it to fit behind the new saddle position.It wouldn't fit due to the clamp profile itself, so I ground it off so the clamp was flat. Therefore it would hold fast on the non-parallel area to the rear of the saddle rails.
This seems to work ok, so now I have a situation as good as it can get.
Test ride required, but that won't happen for a day or two ...... maybe Thursday.

PS:
The finished article.

Yes, that's Mercian behind.

Bmblbzzz wrote:Interesting thread. I like that a question about saddle comfort is solved by foot position!

Me too

I’ve been thinking about this on and off while doing other things today, and I thought of a reason for smaller frames to have steeper seat-tube angles.

It is this: short and tall people ride the same seat posts and saddles, and these combinations usually put the weight-bearing part of the saddle behind the seat post, i.e. there is effectively some set-back.

Since the meaningful angle is the one subtended by the ground and the straight line between the weight-bearing part of the saddle and the bottom bracket, a given set-back has a more relaxing effect on a smaller frame than it does on a larger frame, since the set-back to saddle-height ratio is greater.

If my explanation isn’t clear, try visualising a varying saddle height on my bicycle here, which has the weight-bearing part of the saddle behind the seat post.

I’ll maybe do some sums later to put figures on this set-back effect.

Samuel....I think that what you are talking about is usually expressed like this.....
Think of saddle setback as having 2 components.....
1)....seatpost setback and saddle rail setback....... this setback comes with the design of the saddle and seatpost and is independent of saddle height and seat tube angle
2) ...setback of seatpost top behind BB axle.....this setback varies with both saddle height and seat tube angle and comes from the design of the frame .

Now, if you are comparing the total saddle setback required by riders say 5'0" and 6'6" tall, then its clear that the component #1# setback (the setback due to saddle/seatpost design) is a bigger proportion of the total setback for the short rider than it is for the tall rider.
So to that extent it would be reasonable to steepen the seat tube angle of a small frame to "compensate".

Lets say I'm designing for a height range of riders who will have their seatpost top between 50 and 75 cm above the BB centre, and lets pick 73 degrees for the angle.
I calculate that a 17 degree arc of a circle radius 50cm is about 15cm
and a 17deg. arc of a 75cm circle is about 22cm....so that is the #2# component of the setback that comes from the frames.

The trouble is, I don't know of any "real world" numbers for how far back riders of particular heights want their saddles.
Any saddle at all puts your bum bones so far behind the seatpost that the difference between in-line and layback seatposts looks small, and the difference of one degree in seat tube angle looks vanishingly small, but we know it to be important.
And yet, I don't remember recommending an in-line seatpost to anybody, ever, and i have certainly fitted people say 5'4" on layback seatposts.

My feeling is that you shouldn't steepen the seat tube angle very much at all to "compensate" for the fact that the shortest rider might have the same #1# saddle/seatpost setback as the tallest rider. I'm sorry its just a feeling, I can't think of how to arrive mathematically at the degree of "compensation" you could apply....my feeling is less than a degree.

The Rivendell article lost me when they started talking about Sprinter's blocks, but it had occurred to me that a steep seat tube angle gives a nice short top tube in the geo. table.
Edit....typo

It strikes me that lots of reasons have been given why a steep seat tube is not necessary and lots why a relaxed one is not necessary, but none why either is needed. Which might tend to suggest that both steep and relaxed seat tube angles are fashion. I'm yet to be convinced by anything here that seat tube angle in itself is important; it only becomes so when it affects other dimensions, such as saddle position relative to BB or top tube length.

531colin wrote:Samuel....I think that what you are talking about is usually expressed like this.....
The Rivendell article lost me when they started talking about Sprinter's blocks, but it had occurred to me that a steep seat tube angle gives a nice short top tube in the geo. table.

That bit seemed to be going round in circles. The sprinter goes forward because his knees are in front of his feet? How about long distance runners who start the race standing up? How about every runner having one knee behind its foot at all times? The runner is pushing against the ground, which does not move. A cyclist is pushing against pedals, which do move. If anything on the bike corresponds to the runner's blocks, it would be the bottom bracket, which remains stationary relative to the rider; but that still isn't comparable, because you don't push directly on it.

You only realise the importance of seat tube angle when you can't get your saddle where you want it.
As the overwhelming majority are along the lines of Mick F's Mercian and Paulatic's bike where the saddle is at the rearmost extreme of its adjustment, i conclude that steep seat tube angles are unhelpful.
Of course, if all bike saddles and seatposts were made to give 50mm more setback than is common now, we could all ride even steeper frames.....but I can't see why that would be considered to be "better".